Nonflysch

Nonflysch is a term used in sedimentary geology to describe rock sequences that do not show the flysch facies. Flysch is a characteristic succession of deep-water turbidites and interbedded shales and sandstones developed in the early phase of many orogenic foreland basins. By contrast, nonflysch deposits are those accumulated in the same basins (or in related settings) after the flysch interval or in environments where turbidity currents do not dominate sedimentation. The term can apply to a broad range of lithologies and environments, and it is not tied to a single stratigraphic unit.

Nonflysch sequences commonly include fluvial, deltaic, lacustrine, coastal, or shallow-marine sediments, as well as coal-bearing strata and occasionally volcaniclastic rocks. Lithologies range from sandstones and conglomerates to mudstones and carbonates. The deposits are usually more proximal or nearshore than classic flysch and may show coarsening-upward or channelized architectures consistent with foreland or passive-margin deposition, rather than the interbedded deep-water turbidites of flysch.

In orogenic belts, nonflysch sequences often overlie flysch as part of a molasse unit, representing the post-orogenic stage of basin evolution. Regions with well-documented nonflysch facies include the European Alpine foreland (the molasse sequences overlying flysch), as well as other Carpathian, Dinaride, and continental-margin basins. The distinction between flysch and nonflysch helps geologists interpret tectonics, sediment supply, and climatic changes during the transition from active subduction to foreland basin development.